In-plane deformations of the heme group in native and nonnative oxidized cytochrome c probed by resonance Raman dispersion spectroscopy

摘要

We determined the dispersion of the depolarization ratio (DPR) of several Raman bands of ferricytochrome c at different pH values using low-ionic-strength conditions. The experimental data predominantly cover the pre-resonance and resonance region of Q-band excitation. The selected pH values 7.5, 9.0, 10.0 and 11.0 correspond to the conformational states III, III, IV, and V of the protein. While the existence of the states III, IV, and V is known for nearly 70 years, the nativelike state III* has only recently been obtained from optical measurements at low ionic strength [D. Verbaro, A. Hagarman, J. B. Soffer and R. Schweitzer-Stenner, Biochemistry, 48, 2990, 2009]. We used group-theoretical arguments to obtain in-plane symmetry-lowering deformations from the obtained DPRs of various Raman bands in the high-wavenumber region of the obtained spectra. Thus, we found that a comparatively strong rhombic deformation along the B_(1g) mode, ν_(18), is maintained at all pH values investigated. It most likely arises from static Jahn-Teller distortion of the E-symmetry ground state. While this distortion depends on the existence of a strong sixth ligand, its occurrence is rather independent of the nature of this ligand as long as a low-spin configuration is maintained. The III → III* transition was found to modify A_(1g)-type perturbations of the heme macrocycle. This is likely to reflect a decrease in nonplanar distortions such as ruffling and saddling. This drop in nonplanarity is slightly reversed by the subsequent transitions into states IV and V. Circular dichroism (CD) spectra of the Q-band region suggest that the III → III* transition reduces the electronic contribution to the Q-band splitting, which could reflect either a weakening or a reorientation of the internal electric field in the heme pocket. Our results underscore the relevance of state III* as a thermodynamic intermediate of the alkaline transition between states III and IV.